The reference is to Aristotle, Of memory and recollection, I, 9
"It is clear to which of the several portions of soul memory belongs,
that it belongs where phantasy belongs; and the ordinary objects of
memory are the objects of phantasy, while objects of intellect, which
cannot be without phantasy, are incidental objects of memory." -- The
vis cogitativa, or passive intellect, St Thomas refers to the
same class as phantasy and memory.